Aluminum alloy heat exchanger and method of producing refrigerant tube used for the heat exchanger

ABSTRACT

An aluminum alloy heat exchanger is produced by applying a coating material that is prepared by adding a binder to a mixture of an Si powder and a Zn-containing compound flux powder to a surface of an aluminum alloy refrigerant tube, assembling a bare fin that is formed of an Al—Mn—Zn alloy with the refrigerant tube, and brazing the refrigerant tube and the bare fin by heating in an atmosphere-controlled furnace, the refrigerant tube being an extruded product of an aluminum alloy that comprises 0.5 to 1.7% (mass %, hereinafter the same) of Mn, less than 0.10% of Cu, and less than 0.10% of Si, with the balance being Al and unavoidable impurities, a mixing ratio of the Si powder to the Zn-containing compound flux powder being 10:90 to 40:60, the binder being added in an amount of 5 to 40% based on the total amount of the coating material, the coating material being applied to an outer surface of the refrigerant tube so that the total amount of the Si powder and the Zn-containing compound flux powder is 5 to 30 g/m 2 , the surface of the refrigerant tube subjected to brazing having a potential lower than that of an area of the refrigerant tube that is deeper than a diffusion depth of Si and Zn by 20 to 200 mV, and a potential of the fin being lower than that of a deep area of the refrigerant tube.

BACKGROUND OF THE INVENTION

The present invention relates to an aluminum alloy heat exchanger, and amethod of producing a refrigerant tube used for the heat exchanger.

An aluminum alloy that is lightweight and exhibits excellent thermalconductivity has been normally used for automotive heat exchangers(e.g., evaporator or condenser). Such a heat exchanger has been normallyproduced by applying a fluoride flux to the surface of an aluminum alloyextruded tube (i.e., refrigerant tube), assembling a member (e.g., finmaterial) on the aluminum alloy extruded tube to form a given structure,and brazing the materials in a heating furnace under an inert gasatmosphere, for example.

A refrigerant tube of automotive heat exchangers is normally formedusing an aluminum multi-port extruded tube that has a plurality ofhollow portions that are defined by a plurality of partition walls. Inrecent years, since a reduction in weight of heat exchangers has beendesired to reduce the fuel consumption of automobiles from the viewpointof reducing environmental impact, a refrigerant tube has been reduced inthickness. Therefore, the cross-sectional area of the refrigerant tubehas further decreased, and a several hundred to several thousandextrusion ratio (cross-sectional area of container/cross-sectional areaof extruded product) has been employed. Therefore, a pure aluminummaterial that exhibits excellent extrudability has been used as the tubematerial.

It is expected that heat exchangers will be further reduced in weight,and tubes will be further reduced in weight. In this case, it isnecessary to increase the strength of the tube material. In recentyears, CO₂ (natural refrigerant) has been used instead of fluorocarbonin order to prevent global warming. A CO₂ refrigerant requires a highoperating pressure as compared with a fluorocarbon refrigerant. Thisalso makes it necessary to increase the strength of the tube material.

It is effective to add Si, Cu, Mn, Mg, etc. in order to increase thestrength of the tube material. When the brazing target material containsMg, a fluoride flux that is melted during heating reacts with Mg in thematerial to produce compounds such as MgF₂ and KMgF₃. This reduces theactivity of the flux, so that brazability significantly deteriorates.The operating temperature of a heat exchanger using a CO₂ refrigerantreaches as high as about 150° C. Therefore, intergranular corrosionsusceptibility significantly increases when the material contains Cu.The refrigerant leaks at an early stage when intergranular corrosion hasoccurred, and impairs the function of the tube of the heat exchanger.

Therefore, Si and Mn must be added in order to increase the strength ofthe tube material. When adding Mn and Si to an alloy at a highconcentration, Mn and Si dissolved in the matrix increase thedeformation resistance of the alloy. For example, when a several hundredto several thousand extrusion ratio is employed (e.g., when producing amulti-port extruded tube), the alloy exhibits significantly inferiorextrudability as compared with a pure Al material. An alloy thatrequires a high extrusion ram pressure or has a low critical extrusionrate (i.e., the maximum extrusion rate obtained without causing breakageof the partition wall of the hollow portion of the multi-port tube)exhibits inferior extrudability. An alloy containing Mn and Si at a highconcentration requires a ram pressure higher than that of a pure Almaterial, so that the die tends to break or wear. Moreover, productivitydecreases due to a decrease in critical extrusion rate.

For example, a method that adds Si and Mn that increase strength, andperforms a high-temperature homogenization treatment and alow-temperature homogenization treatment in order to improveextrudability to reduce the amount of solute elements dissolved in thematrix and reduce the deformation resistance has been proposed. In thiscase, since a amount of solute elements is added, an improvement inextrudability (particularly an improvement in extrusion rate) is limitedalthough an increase in strength may be achieved. Specifically, it isdifficult to achieve high strength and high extrudability (i.e.,productivity) at the same time.

A refrigerant leaks from a refrigerant tube of an automotive heatexchanger when perforation corrosion has occurred during use. Therefore,Zn is caused to adhere to the surface of an extruded refrigerant tube bythermal spraying or the like, and is diffused by brazing. A Zn diffusionlayer formed in the surface of the tube serves as a sacrificial anodefor the deeper area, and suppresses corrosion in the thickness directionto increase the perforation life. In this case, a Zn application step(e.g., Zn thermal spraying) is required after extruding the tube.Moreover, a step of applying a fluoride flux required for brazing, or astep of applying a flux to the entire heat exchanger core must beperformed after the Zn application step. This increases the productioncost. Since the tube is not provided with a filler metal, it isnecessary to use a brazing fin that is clad with a filler metal. Thisalso increases cost as compared with the case of using a bare finmaterial that is not clad with a filler metal.

As a method that solves these problems, a method that applies a mixtureof a filler metal powder and a Zn-containing flux powder to the surfaceof an aluminum alloy extruded refrigerant tube has been proposed. Inthis case, since the filler metal, Zn, and the flux can besimultaneously applied by a single step, cost can be reduced. Moreover,since a bare fin material can be used, cost can be further reduced.However, the above method does not necessarily provide the refrigeranttube with strength, extrudability, and corrosion resistance. Arefrigerant tube that contains 0.5 to 1.0% of Si and 0.05 to 1.2% of Mnhas also been proposed. In this case, high strength may be achieved dueto a large amount of solute elements, but an improvement inextrudability (particularly extrusion rate) is limited. Specifically, itis difficult to achieve high strength and high extrudability (i.e.,productivity) in combination.

JP-A-2005-256166, JP-A-2006-255755, JP-A-2006-334614, andJP-A-2004-330233 disclose related-art technologies.

SUMMARY OF THE INVENTION

The present invention was conceived to solve the above problems relatingto automotive heat exchangers and aluminum refrigerant tubes. An objectof the present invention is to provide an aluminum alloy heat exchangerthat exhibits high corrosion resistance, enables a further reduction inweight and cost, and is suitable as an automotive heat exchanger byutilizing a refrigerant tube that is formed of an aluminum alloy thatexhibits excellent strength and corrosion resistance after brazing, andhas improved extrudability. Another object of the present invention isto provide a method of producing a refrigerant tube that improves theextrudability of an aluminum alloy that forms a refrigerant tube usedfor the above heat exchanger.

According to a first aspect of the present invention, there is providedan aluminum alloy heat exchanger that is produced by applying a coatingmaterial that is prepared by adding a binder to a mixture of an Sipowder and a Zn-containing compound flux powder to a surface of analuminum alloy refrigerant tube, assembling a bare fin that is formed ofan Al—Mn—Zn alloy with the refrigerant tube, and brazing the refrigeranttube and the bare fin by heating in an atmosphere-controlled furnace,the refrigerant tube being an aluminum alloy extruded product thatcomprises 0.5 to 1.7% (mass %, hereinafter the same) of Mn, less than0.10% of Cu, and less than 0.10% of Si, with the balance being Al andunavoidable impurities, a mixing ratio of the Si powder to theZn-containing compound flux powder being 10:90 to 40:60, the binderbeing added in an amount of 5 to 40% based on the total amount of thecoating material, the coating material being applied to an outer surfaceof the refrigerant tube so that the total amount of the Si powder andthe Zn-containing compound flux powder is 5 to 30 g/m², the surface ofthe refrigerant tube subjected to brazing having a potential lower thanthat of an area of the refrigerant tube that is deeper than a diffusiondepth of Si and Zn by 20 to 200 mV, and a potential of the fin beinglower than that of the area of the refrigerant tube that is deeper thanthe diffusion depth of Si and Zn.

In the above aluminum alloy heat exchanger, the aluminum alloy extrudedproduct may further comprise at least one of 0.30% or less of Ti, 0.10%or less of Sr, and 0.3% or less of Zr.

In the above aluminum alloy heat exchanger, the Zn-containing compoundflux powder may be KZnF₃.

In the above aluminum alloy heat exchanger, the Al—Mn—Zn alloy maycomprise 0.1 to 1.8% of Mn, 0.8 to 3.0% of Zn, and at least one of 0.1to 1.2% of Si, 0.01 to 0.8% of Fe, 0.05 to 0.5% of Mg, 0.3% or less ofCu, 0.3% or less of Cr, 0.3% or less of Zr, and 0.3% or less of Ti, withthe balance being Al and unavoidable impurities.

In the above aluminum alloy heat exchanger, the Al—Mn—Zn alloy mayfurther comprise at least one of 0.001 to 0.10% of In and 0.001 to 0.10%of Sn.

According to a second aspect of the present invention, there is provideda method of producing a refrigerant tube that is used for the abovealuminum alloy heat exchanger, the method comprising subjecting an ingotof the aluminum alloy that forms the refrigerant tube to ahomogenization heat treatment that holds the ingot at 400 to 650° C. for4 hours or more, and hot-extruding the ingot.

According to a third aspect of the present invention, there is provideda method of producing a refrigerant tube that is used for the abovealuminum alloy heat exchanger, the method comprising subjecting an ingotof the aluminum alloy that forms the refrigerant tube to ahomogenization heat treatment, and hot-extruding the ingot, thehomogenization heat treatment including a first-stage heat treatmentthat holds the ingot at 570 to 650° C. for 2 hours or more, and asecond-stage heat treatment that holds the ingot at 400 to 550° C. for 3hours or more.

According to a fourth aspect of the present invention, there is provideda method of producing a refrigerant tube that is used for the abovealuminum alloy heat exchanger, the method comprising subjecting an ingotof the aluminum alloy that forms the refrigerant tube to ahomogenization heat treatment, and hot-extruding the ingot, thehomogenization heat treatment including a first-stage heat treatmentthat holds the ingot at 570 to 650° C. for 2 hours or more, followed bycooling the ingot to 200° C. or less, and a second-stage heat treatmentthat holds the ingot at 400 to 550° C. for 3 hours or more.

The present invention thus provides an aluminum alloy heat exchangerthat exhibits high corrosion resistance, enables a further reduction inweight and cost, and is suitable as an automotive heat exchanger byutilizing a refrigerant tube that is formed of an aluminum alloy thatexhibits excellent strength and corrosion resistance after brazing, andhas improved extrudability. The present invention thus also provides amethod of producing a refrigerant tube that improves the extrudabilityof an aluminum alloy that forms a refrigerant tube used for the aboveheat exchanger.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The effects and the reasons for limitations of the alloy components ofthe extruded product of the aluminum alloy that forms the refrigeranttube of the aluminum alloy heat exchanger according to the presentinvention are described below.

Mn:

Mn is dissolved in the matrix after brazing the heat exchanger so thatthe strength of the aluminum alloy can be increased as compared with apure aluminum alloy that has been used to form a multi-port extrudedtube for automotive heat exchangers. The addition of Mn decreasesextrudability (particularly the critical extrusion rate) to only a smallextent as compared with the case of adding the same amount of Si, Cu, orMg. When adding an identical amount of Mn, Si, Cu, or Mg to obtain anidentical strength, a decrease in critical extrusion rate is a minimumwhen adding Mn so that high strength and high extrudability (i.e.,productivity) can be achieved in combination. The Mn content ispreferably 0.5 to 1.7%. If the Mn content is less than 0.5%, an increasein strength may occur to only a small extent. If the Mn content exceeds1.7%, extrudability may decrease. The Mn content is more preferably 0.6to 1.5%.

Si:

The Si content is limited to less than 0.10%. This achieves thefollowing effects. The Si powder applied to the surface of therefrigerant tube diffuses into the refrigerant tube during brazing,forms an Al—Mn—Si intermetallic compound with Mn included in thealuminum alloy that forms the refrigerant tube, and precipitates. Thesolid solubility of Mn and Si in the Si diffusion layer of therefrigerant tube decreases due to precipitation, so that the potentialof the Si diffusion layer becomes lower than that of an area deeper thanthe Si diffusion layer (i.e., an area in which Si is not diffused). As aresult, an area of the refrigerant tube from the surface to the bottomof the Si diffusion layer serves as a sacrificial anode layer for anarea that is deeper than the Si diffusion layer, so that the corrosionperforation life in the depth direction can be improved.

If the Si content is 0.10% or more, since an Al—Mn—Si metal compound isinitially present in the aluminum alloy that forms the refrigerant tube,the solid solubility of Mn in the alloy decreases. In this case, even ifthe Si powder applied to the surface diffuses into the alloy duringbrazing, precipitation of Al—Mn—Si intermetallic compounds decreases, sothat the effect of lowering the potential of the Si diffusion layerdecreases. Therefore, an area of the refrigerant tube from the surfaceto the bottom of the Si diffusion layer does not serve as a sacrificialanode layer (i.e., the corrosion perforation life is not improved). TheSi content is more preferably 0.05% or less in order to reliably achievethe above effect.

Cu:

The Cu content is limited to less than 0.10%. This achieves thefollowing effects (1) to (3).

-   (1) Intergranular corrosion can be suppressed during use of a brazed    automotive heat exchanger (particularly at a high temperature). If    the Cu content is 0.10% or more, the operating temperature of a heat    exchanger increases to about 150° C. particularly when using CO₂ as    a refrigerant, so that precipitation of Cu or the like significantly    occurs at the grain boundary. As a result, the intergranular    corrosion susceptibility increases.-   (2) The addition of Cu significantly decreases extrudability as    compared with Mn. It is also necessary to limit the amount of Cu    taking account of a decrease in extrudability.-   (3) It is known that a potential decreases due to the addition of    Zn, and increases due to the addition of Cu. The inventors found    that the potential-increasing effect of Cu predominantly occurs when    Zn coexists with Cu (particularly when the Zn content is low). In    the present invention, a Zn diffusion layer formed during brazing    with the Zn-containing flux powder has a low surface Zn    concentration as compared with a Zn diffusion layer formed during    brazing by Zn thermal spraying, etc. Therefore, if the refrigerant    tube contains 0.10% or more of Cu, the potential-decreasing effect    of the Zn diffusion layer formed by the Zn-containing flux powder is    counterbalanced by the potential-increasing effect of Cu. In this    case, the potential of the surface of the refrigerant tube does not    decrease in spite of the presence of the Zn diffusion layer, so that    a potential gradient cannot be formed such that the surface has a    lower potential and the deep area has a higher potential in the    thickness direction of the refrigerant tube. This makes it difficult    to protect the deep area from corrosion by utilizing the surface as    a sacrificial anode to improve the perforation life. An Si diffusion    layer is present in the surface of the refrigerant tube due to the    applied Si powder, and increases the potential of the surface. When    the Cu content is high, the potential-increasing effect of Cu    becomes completely predominant over the potential-decreasing effect    of the Zn diffusion layer, so that a potential gradient is formed    such that the surface has a higher potential and the deep area has a    lower potential in the thickness direction of the refrigerant tube    along with the potential-increasing effect of the Si diffusion    layer. In this case, since the deep area serves as an anode with    respect to the surface of the refrigerant tube, perforation    corrosion occurs at an early stage. The surface Zn concentration may    be increased by increasing the deposition amount of the    Zn-containing flux powder. However, this increases the thickness of    the resulting film. In this case, the thickness of the film    decreases during brazing due to melting of Si and the flux so that    the distance between the refrigerant tube and the fin material    decreases. Since the above phenomenon occurs over the entire core,    the outer dimension of the core decreases. When the Cu content is    limited to less than 0.10%, the potential of the surface of the    refrigerant tube decreases due to the low-concentration Zn diffusion    layer. Therefore, a potential distribution in the thickness    direction can be formed such that the surface has a lower potential    and the deep area has a higher potential. This protects the deep    area against corrosion by utilizing the surface of the refrigerant    tube as a sacrificial anode. The Cu content is more preferably less    than 0.05%, and still more preferably 0.03% or less.

Ti, Sr, and Zr:

Ti forms a high-Ti-concentration area and a low-Ti-concentration area inthe alloy. These areas are alternately distributed in layers in thedirection of the thickness of the material. Since thelow-Ti-concentration area is preferentially corroded as compared withthe high-Ti-concentration area, corrosion occurs in a layered manner.Therefore, corrosion does not proceed in the thickness direction of thematerial, so that pitting corrosion resistance and intergranularcorrosion resistance are improved. Moreover, the strength of thematerial at room temperature and a high temperature is improved byadding Ti. The Ti content is preferably 0.30% or less. If the Ti contentexceeds 0.30%, coarse crystallized products may be produced duringcasting. This may make it difficult to produce a sound refrigerant tube.

Sr causes the Si powder applied to the surface of the refrigerant tubeto react with Al in the matrix during brazing to produce an Al-Si alloyliquid filler metal, and causes the crystallized eutectic structure tobe refined and dispersed during solidification due to cooling. When theeutectic structure that serves as an anode site on the surface of thematerial is dispersed, corrosion is uniformly dispersed, so that aplanar corrosion configuration is obtained. This improves corrosionresistance. The Sr content is preferably 0.10% or less. If the Srcontent exceeds 0.10%, an Al—Si—Sr compound may be crystallized, so thatthe eutectic structure may not be refined.

Zr increases the size of recrystallized grains when the alloy that formsthe refrigerant tube recrystallizes during brazing. This reduces thegrain boundary density of the matrix, suppresses a phenomenon in whichthe Al—Si alloy liquid filler metal produced by the Si powder applied tothe surface of the refrigerant tube penetrates the grain boundaries ofthe matrix, and suppresses preferential intergranular corrosion. The Zrcontent is preferably 0.30% or less. If the Zr content exceeds 0.30%,coarse crystallized products may be produced during casting. This maymake it difficult to produce a sound refrigerant tube. The effects ofTi, Sr, and Zr can be obtained in combination by adding Ti, Sr, and Zrin combination.

The eroded product of the aluminum alloy that forms the refrigerant tubeof the aluminum alloy heat exchanger according to the present inventionis preferably produced as follows. An aluminum alloy having the abovecomposition is melted, and cast to obtain an ingot. The ingot issubjected to a homogenization treatment that holds the ingot at 400 to650° C. for 4 hours or more, and hot-extruded. The homogenizationtreatment causes coarse crystallized products formed during casting andsolidification to be decomposed or granulated, so that a non-uniformtexture (e.g., segregation layer) produced during casting can behomogenized. When coarse crystallized products or a non-uniform texture(e.g., segregation layer) produced during casting remain during hotextrusion, extrudability may decrease, or the surface roughness of theextruded product may decrease. If the homogenization temperature is lessthan 400° C., the above effects may not be obtained. The above effectsare more easily obtained as the homogenization temperature increases. Ifthe homogenization temperature is more than 650° C., melting may occur.The homogenization temperature is more preferably 430 to 620° C. Thehomogenization time is preferably 10 hours or more in order to achieve asufficient effect. The effect of the homogenization treatment may besaturated (i.e., uneconomical) even if the homogenization treatment isperformed for more than 24 hours. Therefore, the homogenization time ispreferably 10 to 24 hours.

The ingot may be subjected to a high-temperature homogenizationtreatment and a low-temperature homogenization treatment in combination.This further improves hot-extrudability, and reduces aluminum refuse.The term “aluminum refuse” refers to a defect wherein aluminum piecesaccumulated in the die during extrusion are discharged from the die whena given size is reached, and adhere to the surface of the refrigeranttube aluminum extruded product. The high-temperature homogenizationtreatment (first-stage heat treatment) holds the ingot at 570 to 650° C.for 2 hours or more. This treatment causes coarse crystallized productsformed during casting and solidification to be decomposed, granulated,or redissolved. If the treatment temperature is less than 570° C.,redissolution may proceed to only a small extent. It is effective toemploy a high homogenization temperature. However, melting may occur ifthe homogenization temperature is too high. Therefore, thehomogenization temperature is set to be 650° C. or less. Thehomogenization temperature is more preferably 580 to 620° C. Thehomogenization time is preferably 5 to 24 hours. The effect of thehomogenization treatment may be saturated (i.e., uneconomical) even ifthe homogenization treatment is performed for more than 24 hours.

When performing a homogenization treatment (second-stage heat treatment)after the high-temperature homogenization treatment (first-stage heattreatment) at a temperature lower than that of the high-temperaturehomogenization treatment, Mn dissolved in the matrix precipitates, sothat the solid solubility of Mn decreases. This reduces deformationresistance during the subsequent hot extrusion, so that extrudabilitycan be improved. The temperature of the low-temperature homogenizationtreatment (second-stage heat treatment) is preferably 400 to 550° C. Ifthe temperature of the low-temperature homogenization treatment(second-stage heat treatment) is less than 400° C., since only a smallamount of Mn precipitates, the effect of reducing the deformationresistance may be insufficient. If the temperature of thelow-temperature homogenization treatment (second-stage heat treatment)exceeds 550° C., precipitation may occur to only a small extent, so thatthe effect of reducing the deformation resistance may be insufficient.The low-temperature homogenization treatment (second-stage heattreatment) is performed for 3 hours or more. If the treatment time isless than 3 hours, precipitation may not sufficiently occur, so that theeffect of reducing the deformation resistance may be insufficient. Theeffect of the low-temperature homogenization treatment (second-stageheat treatment) may be saturated (i.e., uneconomical) even if thehomogenization treatment is performed for more than 24 hours. Thelow-temperature homogenization treatment (second-stage heat treatment)is preferably performed for 5 to 15 hours. The above two-stagehomogenization treatment is designed so that Mn that has beensufficiently and homogeneously dissolved by the first-stage heattreatment is precipitated by the second-stage heat treatment. Thefirst-stage heat treatment and the second-stage heat treatment need notnecessarily be performed consecutively. Specifically, the second-stageheat treatment may be performed immediately after the first-stage heattreatment, or may be performed after cooling the ingot subjected to thefirst-stage heat treatment to 200° C. or less.

Mixture of Si Powder, Zn-Containing Compound Flux Powder, and Binder

When brazing the aluminum alloy heat exchanger according to the presentinvention, a coating material that is prepared by adding a binder to amixture of an Si powder and a Zn-containing compound flux powder isapplied to the surface of the refrigerant tube. The following effectsare achieved by applying the coating material. Specifically, the Sipowder reacts with Al of the matrix of the refrigerant tube duringbrazing to produce an Al—Si filler metal, so that a fin material or aheader material can be bonded to the refrigerant tube. The Zn-containingflux decomposes into the flux and Zn during brazing. The flux enablesbrazing, and Zn diffuses into the refrigerant tube to form a Zndiffusion layer. A potential gradient can thus be formed so that thesurface of the refrigerant tube has a lower potential and the deep areaof the refrigerant tube has a higher potential. Therefore, the deep areacan be protected against corrosion by utilizing the surface area as asacrificial anode. The binder improves adhesion when causing the mixedpowder to adhere to the refrigerant tube. The particle size of the Sipowder included in the mixture of the Si powder and the Zn-containingcompound flux powder is preferably 100 μm or less, more preferably 30 μmor less, and still more preferably 15 μm or less. The fluidity of theAl—Si liquid filler metal produced during brazing is improved as theparticle size of the Si powder decreases. Moreover, erosion of thematrix is suppressed as the particle size of the Si powder decreases. Itis preferable that the Zn-containing compound flux powder have anaverage particle size of about 5 μm. For example, KZnF₃ is used as theZn-containing compound flux powder.

The mixing ratio of the Si powder to the Zn-containing compound fluxpowder is preferably 10:90 to 40:60. If the mixing ratio is less than10:90 (i.e., the amount of the Si powder is less than 10%), a sufficientliquid filler metal may not be produced during brazing, so that bondingfailure may occur. If the mixing ratio is more than 40:60 (i.e., theamount of the Si powder is more than 40%), the amount of Zn diffusedinto the refrigerant tube may be insufficient. Moreover, brazability maydeteriorate due to a decrease in the amount of flux.

When applying the mixture to the surface of the refrigerant tube,adhesion is improved by applying the mixture as a coating material thatis prepared by adding a binder (e.g., a resin that volatilizes duringheating for brazing) to the mixture. For example, an acrylic resin isused as the binder. The binder is used in an amount of 5 to 40% based onthe total amount of the coating material. If the amount of the binder isless than 5% based on the total amount of the coating material, themixture may easily separated from the surface of the refrigerant tube.If the amount of the binder is more than 40% based on the total amountof the coating material, brazability may deteriorate.

The mixture of the Si powder and the Zn-containing compound flux powderis preferably applied in an amount of 5 to 30 g/m². If the amount of themixture applied is less than 5 g/m², the amount of Zn that adheres tothe surface of the refrigerant tube may be insufficient. If the amountof the mixture applied is more than 30 g/m², the amount of filler metalproduced may increase, so that melting or dissolution of the fin or thematrix may easily occur. Moreover, since the thickness of the filmbetween the refrigerant tube and the fin material increases, thedimensions of the entire core may decrease if the film is melted duringbrazing and is reduced in thickness. The mixture may be applied to therefrigerant tube by roll coating.

Potential Difference Between Surface and Deep Area of Refrigerant Tubeand Relationship with Potential of Fin Material

In the aluminum alloy heat exchanger according to the present invention,the surface of the refrigerant tube has a potential lower than that ofan area of the refrigerant tube that is deeper than a diffusion depth ofSi and Zn by 20 to 200 mV, and the potential of the fin is lower thanthat of the deep area of the refrigerant tube. Therefore, the surface ofthe refrigerant tube serves as a sacrificial anode with respect to thedeep area so that the deep area can be cathodically protected. If thepotential difference is smaller than 20 mV, a sufficient sacrificialanode effect may not be obtained. If the potential difference is largerthan 200 mV, the corrosion rate of the surface area increases, so thatthe sacrificial anode may be quickly exhausted. It is also importantthat the potential of the fin be lower than that of the deep area of therefrigerant tube. If the potential of the fin is higher than that of thedeep area of the refrigerant tube, the fin serves as a cathode withrespect to the refrigerant tube, so that corrosion of the refrigeranttube is promoted. Therefore, the potential of the fin must be lower thanthat of the deep area of the refrigerant tube.

When producing a heat exchanger using the refrigerant tube according tothe present invention, defective brazing that may occur at a jointbetween the refrigerant tube and a header material can be suppressed.Specifically, the refrigerant tube and the header material are mainlybonded via a filler metal applied to the header material. However, theSi powder adheres to the surface of the refrigerant tube, and the jointis covered with a liquid filler metal that is produced by the Si powderand the surface area of the refrigerant tube that are melted duringbrazing. Therefore, the filler metal of the header material communicateswith the liquid filler metal on the surface of the refrigerant tube(i.e., flows freely). The refrigerant tube is bonded to the fin on theside opposite to the header, and the filler metal of the header materialmoves along the surface of the refrigerant tube, and reaches the jointwith the fin due to surface tension. Therefore, the amount of fillermetal becomes insufficient at the joint between the header and therefrigerant tube, so that defective brazing occurs. In particular,defective brazing occurs when using a refrigerant tube formed of a purealuminum alloy or an alloy produced by adding Cu to a pure aluminumalloy. On the other hand, when forming a refrigerant tube using thealuminum alloy according to the present invention, defective brazingdoes not occur at the joint between the refrigerant tube and the headermaterial even when the header material is provided with the same amountof filler metal as in the case of using the refrigerant tube formed ofthe above alloy. Specifically, since an Al—Mn precipitate (resistance)is present on the surface of the refrigerant tube aluminum alloyaccording to the present invention, the wettability of the liquid fillermetal with the surface of the aluminum alloy can be suppressed ascompared with a pure aluminum alloy or an alloy produced by adding Cu toa pure aluminum alloy. This makes it possible to prevent a situation inwhich the filler metal of the header material moves along the surface ofthe refrigerant tube and flows into the joint with the fin. In thepresent invention, since the refrigerant tube is bonded to the finmaterial through the mixture of the Si powder and the Zn-containing fluxthat is applied to the surface of the refrigerant tube, it is possibleto reduce the Zn concentration of the fillet at the joint with the finmaterial as compared with the case of applying Zn to the surface of therefrigerant tube by thermal spraying or the like. Therefore,preferential corrosion of the fillet at the joint with the fin can besuppressed, so that removal of the fin can be prevented.

The effects and the reasons for limitations of the alloy components ofthe aluminum alloy that forms the bare fin material of the aluminumalloy heat exchanger according to the present invention are describedbelow.

Mn:

Mn improves the strength of the fin material. The Mn content ispreferably 0.1 to 1.8%. If the Mn content is less than 0.1%, the effectmay be insufficient. If the Mn content exceeds 1.8%, coarse crystallizedproducts may be produced during casting. This may make it difficult toproduce a sound fin material. The Mn content is more preferably 0.8 to1.7%.

Zn:

Zn decreases the potential of the fin material. The Zn content ispreferably 0.8 to 3.0%. If the Zn content is less than 0.8%, asufficient potential-decreasing effect may not be obtained. If the Zncontent exceeds 3.0%, the potential of the fin material is sufficientlydecreased, but the self-corrosion resistance of the fin material maydecrease. Moreover, since the potential difference between the fin andthe deep area of the refrigerant tube increases, the fin (anode) may beconsumed at an early stage due to corrosion in an environment in whichthe material is always exposed to a high-conductivity liquid. The Zncontent is more preferably 1.0 to 2.5%.

Si, Fe, Cu, Mg, Cr, Zr, and Ti:

Si improves the strength of the fin material. The Si content ispreferably 0.1 to 1.2%. If the Si content is less than 0.1%, the effectmay be insufficient. If the Si content exceeds 1.2%, the melting pointof the fin material may decrease, so that local melting may occur duringbrazing. The Si content is more preferably 0.2 to 0.6%.

Fe improves the strength of the fin material. The Fe content ispreferably 0.01 to 0.8%. If the Fe content is less than 0.01%, theeffect may be insufficient. If the Fe content exceeds 0.8%, the amountof Al—Fe compounds produced may increase, so that the self-corrosionresistance of the fin material may decrease. The Fe content is morepreferably 0.1 to 0.7%.

Mg improves the strength of the fin material. The Mg content ispreferably 0.05 to 0.5%. If the Mg content is less than 0.05%, theeffect may be insufficient. If the Mg content exceeds 0.5%, Mg reactswith a fluoride flux to produce magnesium fluoride during brazing in aninert gas atmosphere using a fluoride flux. As a result, brazability maydecrease, and the appearance of the brazed area may deteriorate. The Mgcontent is more preferably 0.05 to 0.3%, and still more preferably 0.05to 0.15%.

Cu improves the strength of the fin material. The Cu content ispreferably 0.3% or less. If the Cu content exceeds 0.3%, the potentialof the fin material may increase, so that the corrosion resistance ofthe refrigerant tube may be impaired. Moreover, the self-corrosionresistance of the fin material may decrease.

Cr and Zr increase the grain size after brazing, and reduce buckling ofthe fin during brazing. The Cr content and the Zr content are preferably0.3% or less. If the Cr content or the Zr content exceeds 0.3%, coarsecrystallized products may be produced during casting. This may make itdifficult to produce a sound fin material.

Ti forms a high-Ti-concentration area and a low-Ti-concentration area inthe alloy. These areas are alternately distributed in layers in thedirection of the thickness of the material. Since thelow-Ti-concentration area is preferentially corroded as compared withthe high-Ti-concentration area, corrosion occurs in a layered manner.Therefore, corrosion does not proceed in the thickness direction of thematerial. As a result, pitting corrosion resistance and intergranularcorrosion resistance are improved. Moreover, the strength of thematerial at room temperature and a high temperature is improved byadding Ti. The Ti content is preferably 0.3% or less. If the Ti contentexceeds 0.3%, coarse crystallized products may be produced duringcasting. This may make it difficult to produce a sound fin material.

In and Sn:

In and Sn decrease the potential of the fin material with a small amountof addition. In and Sn exhibit a sacrificial anode effect for therefrigerant tube, and prevent pitting corrosion of the refrigerant tube.The In content and the Sn content are preferably 0.001 to 0.1%. If theIn content or the Sn content is less than 0.001%, the effect may beinsufficient. If the In content or the Sn content exceeds 0.1%, theself-corrosion resistance of the fin material may decrease.

The heat exchanger according to the present invention may be produced byassembling the refrigerant tube and the fin material having the abovecomposition, and brazing the refrigerant tube and the fin material by anormal method. The production method is not particularly limited. Theheat exchanger according to the present invention exhibits excellentcorrosion resistance, and exhibits excellent durability even wheninstalled in an automobile that is subjected to a severe corrosiveenvironment, for example. The heating method and the structure of theheating furnace used when subjecting the aluminum alloy that forms therefrigerant tube to the homogenization treatment are not particularlylimited. The shape of the aluminum alloy extruded product that forms therefrigerant tube is not particularly limited. The extrusion shape isdetermined depending on the application (e.g., the shape of the heatexchanger). Since the material has excellent extrudability, the materialmay be extruded using a multi-cavity die having a hollow shape. Forexample, the refrigerant tube for heat exchangers is normally assembledwith another member (e.g., fin material or header material), followed bybrazing. The brazing atmosphere, the heating temperature, the heatingtime, and the brazing method are not particularly limited. The finmaterial is normally produced by producing an ingot by semi-continuouscasting, and subjecting the ingot to hot rolling, cold rolling, processannealing, and cold rolling. Note that process annealing may be omitted.A hot-rolled sheet may be directly obtained from a molten metal bycontinuous casting and rolling, and may be cold-rolled.

EXAMPLES

An aluminum alloy extruded product for a refrigerant tube was producedas follows. A billet of an aluminum alloy (Alloys A to L) having acomposition shown in Table 1 or an aluminum alloy (Alloys M to T) havinga composition shown in Table 2 was cast. Alloy T has been widely used.The resulting billet was subjected to the following tests 1, 2, and 3.In Table 2, a value that does not meet the requirements of the presentinvention is underlined.

Test 1

The cast billet was homogenized at 600° C. for 10 hours, andhot-extruded to obtain a multi-port tube. The critical extrusion rateratio (relative ratio with respect to the critical extrusion rate ofAlloy T) during extrusion was determined. The results are shown inTables 3 and 4. A case where the critical extrusion rate ratio was morethan 1.0 was evaluated as “Good”, and a case where the criticalextrusion rate ratio was less than 1.0 was evaluated as “Bad”(extrudability evaluation).

Test 2

The multi-port tube extruded in Test 1 was brazed. The multi-port tubewas heated to 600° C. in a nitrogen gas atmosphere at an averagetemperature increase rate of 50° C./min, held for 3 minutes, and cooledto room temperature. The multi-port tube was then subjected to a tensiletest at room temperature. The results (tensile strength) are shown inTables 3 and 4. A case where the tensile strength was higher than thatof Alloy T was evaluated as “Good”, and a case where the tensilestrength was lower than that of Alloy T was evaluated as “Bad”(evaluation of strength after brazing).

Test 3

The billets of Alloys C and D were homogenized under conditions shown inTables 5 and 6, and hot-extruded to obtain multi-port tubes. Thecritical extrusion rate ratio (relative ratio with respect to thecritical extrusion rate of Alloy T) was determined. The temperatureincrease rate was 50° C./h. The temperature decrease rate whensuccessively performing the first-stage heat treatment and thesecond-stage heat treatment was 25° C./h. The billet was allowed to coolafter the second-stage heat treatment. The results (critical extrusionrate ratio) are shown in Tables 5 and 6. A case where the criticalextrusion rate ratio was more than 1.0 was evaluated as “Good”, and acase where the critical extrusion rate ratio was less than 1.0 wasevaluated as “Bad” (extrudability evaluation).

TABLE 1 Composition (mass %) Alloy Si Fe Cu Mn Ti Sr Zr A 0.05 0.15 00.5 0 0 0 B 0.05 0.15 0 1.7 0 0 0 C 0.05 0.15 0 1.0 0 0 0 D 0.05 0.15 00.7 0 0 0 E 0.05 0.15 0 0.7 0.15 0 0 F 0.05 0.15 0 0.7 0 0.03 0 G 0.050.15 0 0.7 0 0 0.15 H 0.05 0.15 0 0.7 0.15 0.03 0 I 0.05 0.15 0 0.7 00.03 0.15 J 0.05 0.15 0 0.7 0.15 0 0.15 K 0.05 0.15 0 0.7 0.15 0.03 0.15L 0.05 0.15 0.03 0.7 0 0 0

TABLE 2 Composition (mass %) Alloy Si Fe Cu Mn Ti Sr Zr M 0.05 0.15 00.4 0 0 0 N 0.05 0.15 0 1.8 0 0 0 O 0.05 0.15 0 0.7   0.35 0 0 P 0.050.15 0 0.7 0   0.20 0 Q 0.05 0.15 0 0.7 0 0   0.35 R 0.05 0.15   0.150.7 0 0 0 S 0.05 0.15   0.01  0.01 0 0 0 T 0.05 0.15   0.4 0.1 0 0 0

TABLE 3 Extrudability Brazability Critical Tensile Strength extrusionstrength after after Alloy rate ratio Evaluation brazing (MPa) brazing A1.41 Good 75 Good B 1.00 Good 115 Good C 1.17 Good 100 Good D 1.33 Good80 Good E 1.29 Good 83 Good F 1.29 Good 80 Good G 1.29 Good 80 Good H1.15 Good 84 Good I 1.15 Good 81 Good J 1.15 Good 84 Good K 1.10 Good 84Good L 1.30 Good 82 Good

TABLE 4 Extrudability Brazability Critical Tensile Strength extrusionstrength after after Alloy rate ratio Evaluation brazing (MPa) brazing M1.42 Good 70 Bad N 0.90 Bad 120 Good O 0.95 Bad 90 Good P 0.95 Bad 85Good Q 0.95 Bad 85 Good R 0.95 Bad 87 Good S 1.58 Good 60 Bad T 1.00 —75 —

TABLE 5 First-stage Cooling to Second-stage heat treatment roomtemperature heat treatment Extrudability Temperature Time beforesecond-stage Temperature Time Critical extrusion Alloy (C. °) (h) heattreatment (C. °) (h) rate ratio Evaluation C 600 10 — — 1.17 Good D 60010 — — 1.33 Good C 600 10 None 500 10 1.25 Good C 600 10 Cooled 500 101.27 Good D 600 10 None 500 10 1.45 Good

TABLE 6 First-stage Cooling to Second-stage heat treatment roomtemperature heat treatment Extrudability Temperature Time beforesecond-stage Temperature Time Critical extrusion Alloy (C. °) (h) heattreatment (C. °) (h) rate ratio Evaluation C 600 2 — — 0.95 Bad C 350 10— — 0.90 Bad C 600 1 None 500 2 0.85 Bad C 500 10 None 450 10 0.90 Bad T600 10 — — 1.00 —

As shown in Table 3 and 4, Alloys A to L according to the presentinvention exhibited excellent extrudability and brazability. On theother hand, Alloys M to S that do not meet the requirements of thepresent invention exhibited inferior extrudability or brazability.

When homogenizing Alloys C and D according to the present inventionunder the conditions shown in Tables 5 and 6, excellent extrudabilitywas obtained when homogenizing the alloy under the conditions(conditions shown in Table 5) according to the present invention. On theother hand, inferior extrudability was obtained when homogenizing thealloy under conditions that do not meet the requirements of the presentinvention.

As an aluminum alloy for a fin material, a slab of an aluminum alloy(Alloys a to 1) having a composition shown in Table 7 or an aluminumalloy (Alloys m to x) having a composition shown in Table 8 was cast.The slab was homogenized, hot-rolled, and cold-rolled to obtain a finmaterial having a thickness of 0.1 mm. The fin material was thencorrugated (fin pitch: 3 mm, fin height: 7 mm). In Tables 7 and 8, avalue that does not meet the requirements of the present invention isunderlined.

TABLE 7 Composition (mass %) Alloy Si Fe Cu Mn Zn Others a 0.05 0.15 01.2 0.8 b 0.05 0.15 0 1.2 3.0 c 0.05 0.15 0 1.2 1.0 d 0.05 0.15 0.15 1.22.5 e 0.5 0.15 0 1.2 1.0 f 0.5 0.15 0.15 1.2 2.5 g 0.05 0.15 0 1.2 1.0Mg: 0.1 h 0.05 0.15 0 1.2 1.0 Cr: 0.15 i 0.05 0.15 0 1.2 1.0 Zr: 0.15 j0.05 0.15 0 1.2 1.0 Ti: 0.15 k 0.05 0.15 0 1.2 1.0 In: 0.05 l 0.05 0.150 1.2 1.0 S,: 0.05

TABLE 8 Composition (mass %) Alloy Si Fe Cu Mn Zn Others m 0.05 0.15 01.2 0.3 n 0.05 0.15 0 1.2 3.5 o 1.3  0.15 0 1.2 1.0 p 0.05 0.15 0 2.01.0 q 0.05 1.0  0 1.2 1.0 r 0.05 0.15 0 1.2 1.0 Mg: 0.6 s 0.05 0.15  0.5 1.2 1.0 t 0.05 0.15 0 1.2 1.0 Cr: 0.35 u 0.05 0.15 0 1.2 1.0Zr: 0.35 v 0.05 0.15 0 1.2 1.0 Ti: 0.35 w 0.05 0.15 0 1.2 1.0 In: 0.15 x0.05 0.15 0 1.2 1.0 Sn: 0.15

A coating material was prepared by adding an acrylic resin binder to amixture of an Si powder and a KZnF₃ powder (the mixing ratio is shown inTables 9 and 10). The coating material was applied to the surface of theabove multi-port tube (aluminum alloy multi-port extruded tube forrefrigerant tube) (indicated by the alloy reference symbol in Tables 9and 10) by roll coating in an amount shown in Tables 9 and 10. Themulti-port tube and the corrugated fin (indicated by the alloy referencesymbol in Tables 9 and 10) were assembled (see Tables 9 and 10), andbrazed to obtain a heat exchanger core.

A case where the heat exchanger core was produced without any problemwas evaluated as “Good”, and a case where a problem occurred whenproducing the heat exchanger core was evaluated as “Bad” (evaluation ofheat exchanger core production). The results are shown in Table 9 and10. The multi-port tube was homogenized at 600° C. for 10 hours. Whenbrazing the multi-port tube and the fin, the multi-port tube and the finwere heated to 600° C. in a nitrogen gas atmosphere at an averagetemperature increase rate of 50° C./min, held for 3 minutes, and cooledto room temperature. The resulting heat exchanger core was subjected tothe following tests 4, 5, 6, and 7.

TABLE 9 Coating Acrylic resin Heat exchange Refrigerant tube Si KZnF₃binder Amount Fin material Problem during Core production core No. Alloy(%) (%) (%) (/m³) Alloy core production state 1 A 20 60 20 13 c NoneGood 2 B 20 60 20 13 c None Good 3 C 20 60 20 13 c None Good 4 D 20 6020 13 c None Good 5 E 20 60 20 13 c None Good 6 F 20 60 20 13 c NoneGood 7 G 20 60 20 13 c None Good 8 H 20 60 20 13 c None Good 9 I 20 6020 13 c None Good 10 J 20 60 20 13 c None Good 11 K 20 60 20 13 c NoneGood 12 L 20 60 20 13 c None Good 13 D 20 60 20 13 a None Good 14 D 2060 20 13 b None Good 15 D 20 60 20 13 d None Good 16 D 20 60 20 13 eNone Good 17 D 20 60 20 13 f None Good 18 D 20 60 20 13 g None Good 19 D20 60 20 13 h None Good 20 D 20 60 20 13 i None Good 21 D 20 60 20 13 jNone Good 22 D 20 60 20 13 k None Good 23 D 20 60 20 13 l None Good 24 D20 60 10 13 c None Good

TABLE 10 Coating Refrigerant Acrylic resin Core Heat exchange tube SiKZnF₃ binder Amount Fin material Problem during production core No.Alloy (%) (%) (%) (g/m³) Alloy core production state 25 D 20 60 20 13 mNone Good 26 D 20 60 20 13 n None Good 27 D 20 60 20 13 o Fin was meltedduring brazing Bad 28 D 20 60 20 13 p Fin broke during forming Bad 29 D20 60 20 13 q None Good 30 D 20 60 20 13 r Fin was not bonded duringbrazing Bad 31 D 20 60 20 13 s None Good 32 D 20 60 20 13 t Fin brokeduring forming Bad 33 D 20 60 20 13 u Fin broke during forming Bad 34 D20 60 20 13 v Fin broke during forming Bad 35 D 20 60 20 13 w None Good36 D 20 60 20 13 x None Good 37 D 20 60 20 4 c Fin was not bonded duringbrazing Bad 38 D 20 60 20 25 c Core dimensions decreased Bad 39 D 24 733 13 c Coating separation Bad 40 D 16 49 35 13 c Defective brazing Bad41 D 5 75 20 13 c Fin was not bonded during brazing Bad 42 D 45 35 20 13c Defective brazing Bad 43 T 20 60 20 13 c None Good 44 T 10 85 5 20 cNone Good

Test 4

The heat exchanger core was subjected to a leakage test to determine thepresence or absence of leakage due to defective brazing at the jointbetween the header and the refrigerant tube. The results are shown inTables 11 and 12.

Test 5

The heat exchanger core was heated at 150° C. for 120 hours(high-temperature usage simulation), and subjected to an intergranularcorrosion test in accordance with ISO 11846 (Method B). The results areshown in Tables 13 and 14.

Test 6

The Zn concentration and the Zn diffusion depth of the surface of therefrigerant tube of the heat exchanger core, the potentials of thesurface and the deep area of the refrigerant tube, the potentialdifference between the surface and the deep area of the refrigeranttube, the potential of the fin material, the potential differencebetween the surface of the refrigerant tube and the fin material, andthe potential difference between the deep area of the refrigerant tubeand the fin material were measured. The Zn concentration and the Zndiffusion depth of the surface of the refrigerant tube were determinedby filling the cross section of the core with a resin, and calculatingthe Zn concentration and the Zn diffusion depth from the EPMA lineanalysis results in the thickness direction. A depth at which the Znconcentration was 0.01% was taken as the Zn diffusion depth. Thepotential of the surface of the refrigerant tube and the potential ofthe surface of the fin material were measured directly after brazing.The potential of the deep area of the refrigerant tube was determined byfacing the refrigerant tube to a depth of 150 μm from the surface, andmeasuring the potential of an area in which Zn diffusion did not occur.When measuring the potential of the material, the material was immersedin a 5% NaCl aqueous solution (the pH was adjusted to 3 using aceticacid) for 24 hours. The average value of stable measured values obtainedafter immersing the material for 10 hours or more was employed. Asaturated calomel electrode was used as a reference electrode. Theresults are shown in Tables 15 and 16.

Test 7

The heat exchanger core was subjected to the SWAAT test and the CCT testspecified by ASTM-G85-Annex A3 for 1000 hours. In the CCT test, a 5%salt solution (the pH was adjusted to 3 using acetic acid) was used as atest solution. After spraying the test solution onto the heat exchangercore at 35° C. (atmospheric temperature) for 2 hours, the heat exchangercore was dried at 60° C. for 4 hours, and wetted at 50° C. for 2 hoursat a relative humidity of 95% or more. The above cycle was repeated. Themaximum corrosion depth of the refrigerant tube (tube) and the corrosionstate of the fin after the test are shown in Tables 17 and 18. A casewhere the maximum corrosion depth of the refrigerant tube was 0.05 mm orless was evaluated as “Excellent”, a case where the maximum corrosiondepth of the refrigerant tube was more than 0.05 mm and 0.10 mm or lesswas evaluated as “Good”, a case where the maximum corrosion depth of therefrigerant tube was more than 0.10 mm and 0.20 mm or less was evaluatedas “Fair”, and a case where the maximum corrosion depth of therefrigerant tube was more than 0.20 mm was evaluated as “Bad”. A casewhere the fin was corroded to only a small extent was evaluated as“Excellent”, a case where the fin was slightly corroded was evaluated as“Good”, a case where the fin was corroded to some extent was evaluatedas “Fair”, and a case where the fin was significantly corroded wasevaluated as “Bad”.

TABLE 11 Coating Leakage at Heat exchange core Refrigerant tube Si KZnF₃Acrylic resin binder Amount Fin material header/refrigerant No. Alloy(%) (%) (%) (g/m³) Alloy tube joint 1 A 20 60 20 13 c None 2 B 20 60 2013 c None 3 C 20 60 20 13 c None 4 D 20 60 20 13 c None 5 E 20 60 20 13c None 6 F 20 60 20 13 c None 7 G 20 60 20 13 c None 8 H 20 60 20 13 cNone 9 I 20 60 20 13 c None 10 J 20 60 20 13 c None 11 K 20 60 20 13 cNone 12 L 20 60 20 13 c None 13 D 20 60 20 13 a None 14 D 20 60 20 13 bNone 15 D 20 60 20 13 d None 16 D 20 60 20 13 e None 17 D 20 60 20 13 fNone 18 D 20 60 20 13 g None 19 D 20 60 20 13 h None 20 D 20 60 20 13 iNone 21 D 20 60 20 13 j None 22 D 20 60 20 13 k None 23 D 20 60 20 13 lNone 24 D 20 60 10 13 c None

TABLE 12 Coating Leakage at Heat exchange core Refrigerant tube Si KZnF₃Acrylic resin binder Amount Fin material header/refrigerant No. Alloy(%) (%) (%) (g/m³) Alloy tube joint 25 D 20 60 20 13 m None 26 D 20 6020 13 n None 27 D 20 60 20 13 o None 28 D 20 60 20 13 p None 29 D 20 6020 13 q None 30 D 20 60 20 13 r None 31 D 20 60 20 13 s None 32 D 20 6020 13 t None 33 D 20 60 20 13 u None 34 D 20 60 20 13 v None 35 D 20 6020 13 w None 36 D 20 60 20 13 x None 37 D 20 60 20 4 c None 38 D 20 6020 25 c None 39 D 24 73 3 13 c None 40 D 16 49 35 13 c None 41 D 5 75 2013 c None 42 D 45 35 20 13 c None 43 T 20 60 20 13 c

44 T 10 85 5 20 c

TABLE 13 Coating Heat exchange core Refrigerant tube Si KZnF₃ Acrylicresin binder Amount Fin material Intergranular No. Alloy (%) (%) (%)(g/m³) Alloy corrosion 1 A 20 60 20 13 c None 2 B 20 60 20 13 c None 3 C20 60 20 13 c None 4 D 20 60 20 13 c None 5 E 20 60 20 13 c None 6 F 2060 20 13 c None 7 G 20 60 20 13 c None 8 H 20 60 20 13 c None 9 I 20 6020 13 c None 10 J 20 60 20 13 c None 11 K 20 60 20 13 c None 12 L 20 6020 13 c None 13 D 20 60 20 13 a None 14 D 20 60 20 13 b None 15 D 20 6020 13 d None 16 D 20 60 20 13 e None 17 D 20 60 20 13 f None 18 D 20 6020 13 g None 19 D 20 60 20 13 h None 20 D 20 60 20 13 i None 21 D 20 6020 13 j None 22 D 20 60 20 13 k None 23 D 20 60 20 13 l None 24 D 20 6010 13 c None

TABLE 14 Coating Heat exchange core Refrigerant tube Si KZnF₃ Acrylicresin binder Amount Fin material Intergranular No. Alloy (%) (%) (%)(g/m³) Alloy corrosion 25 D 20 60 20 13 m None 26 D 20 60 20 13 n None27 D 20 60 20 13 o None 28 D 20 60 20 13 p None 29 D 20 60 20 13 q None30 D 20 60 20 13 r None 31 D 20 60 20 13 s None 32 D 20 60 20 13 t None33 D 20 60 20 13 u None 34 D 20 60 20 13 v None 35 D 20 60 20 13 w None36 D 20 60 20 13 x None 37 D 20 60 20 4 c None 38 D 20 60 20 25 c None39 D 24 73 3 13 c None 40 D 16 49 35 13 c None 41 D 5 75 20 13 c None 42D 45 35 20 13 c None 43 T 20 60 20 13 c Significant 44 T 10 85 5 20 cSignificant

TABLE 15 Refrigerant Tube Tube deep Heat Re- Coating tube Zn Refrigeranttube potential Fin surface/fin area/fin ex- friger- Acrylic Fin SurfaceDiffu- Deep material material material change ant resin mate- concen-sion Surface area Potential potential potential potential core tube SiKZnF₃ binder Amount rial tration depth (mV vs. (mV vs. difference (mVvs. difference difference No. Alloy (%) (%) (%) (g/m³) Alloy (mass %)(μm) SCE) SCE) (mV) SCE) (mV) (mV) 1 A 20 60 20 13 c 1.5 100 −845 −75095 −780 −65 30 2 B 20 60 20 13 c 1.5 100 −815 −720 95 −780 −35 60 3 C 2060 20 13 c 1.5 100 −825 −730 95 −780 −45 50 4 D 20 60 20 13 c 1.5 100−835 −740 95 −780 −55 40 5 E 20 60 20 13 c 1.5 100 −835 −740 95 −780 −5540 6 F 20 60 20 13 c 1.5 100 −835 −740 95 −780 −55 40 7 G 20 60 20 13 c1.5 100 −835 −740 95 −780 −55 40 8 H 20 60 20 13 c 1.5 100 −835 −740 95−780 −55 40 9 I 20 60 20 13 c 1.5 100 −835 −740 95 −780 −55 40 10 J 2060 20 13 c 1.5 100 −835 −740 95 −780 −55 40 11 K 20 60 20 13 c 1.5 100−835 −740 95 −780 −55 40 12 L 20 60 20 13 c 1.5 100 −830 −730 100 −780−50 50 13 D 20 60 20 13 a 1.5 100 −835 −740 95 −760 −75 20 14 D 20 60 2013 b 1.5 100 −835 −740 95 −900 65 160 15 D 20 60 20 13 d 1.5 100 −835−740 95 −800 −35 60 16 D 20 60 20 13 e 1.5 100 −835 −740 95 −770 −65 3017 D 20 60 20 13 f 1.5 100 −835 −740 95 −790 −45 50 18 D 20 60 20 13 g1.5 100 −835 −740 95 −780 −55 40 19 D 20 60 20 13 h 1.5 100 −835 −740 95−780 −55 40 20 D 20 60 20 13 i 1.5 100 −835 −740 95 −780 −55 40 21 D 2060 20 13 j 1.5 100 −835 −740 95 −780 −55 40 22 D 20 60 20 13 k 1.5 100−835 −740 95 −820 −15 80 23 D 20 60 20 13 l 1.5 100 −835 −740 95 −820−15 80 24 D 20 60 10 13 c 1.5 100 −835 −740 95 −780 −55 40

TABLE 16 Refrigerant Tube Tube deep Heat Re- Coating tube Zn Refrigeranttube potential Fin surface/fin area/fin ex- friger- Acrylic Fin SurfaceDiffu- Deep material material material change ant resin mate- concen-sion Surface area Potential potential potential potential core tube SiKZnF₃ binder Amount rial tration depth (mV vs. (mV vs. difference (mVvs. difference difference No. Alloy (%) (%) (%) (g/m³) Alloy (mass %)(μm) SCE) SCE) (mV) SCE) (mV) (mV) 25 D 20 60 20 13 m 1.5 100 −835 −74095 −730 −105 −10 26 D 20 60 20 13 n 1.5 100 −835 −740 95 −950 115 210 27D 20 60 20 13 o 1.5 100 −835 −740 95 −750 −85 10 28 D 20 60 20 13 p 1.5100 −835 −740 95 −720 −115 −20 29 D 20 60 20 13 q 1.5 100 −835 −740 95−780 −55 40 30 D 20 60 20 13 r 1.5 100 −835 −740 95 −780 −55 40 31 D 2060 20 13 s 1.5 100 −835 −740 95 −700 −135 −40 32 D 20 60 20 13 t 1.5 100−835 −740 95 −780 −55 40 33 D 20 60 20 13 u 1.5 100 −835 −740 95 −780−55 40 34 D 20 60 20 13 v 1.5 100 −835 −740 95 −780 −55 40 35 D 20 60 2013 w 1.5 100 −835 −740 95 −820 −15 80 36 D 20 60 20 13 x 1.5 100 −835−740 95 −820 −15 80 37 D 20 60 20 4 c 0.2 45 −760 −740 20 −780 20 40 38D 20 60 20 25 c 2.4 140 −880 −740 140 −780 −100 40 39 D 24 73 3 13 c 1.7105 −845 −740 105 −780 −65 40 40 D 16 49 35 13 c 1.0 80 −780 −740 40−780 0 40 41 D 5 75 20 13 c 1.8 110 −850 −740 110 −780 −70 40 42 D 45 3520 13 c 0.5 60 −770 −740 30 −780 10 40 43 T 20 60 20 13 c 1.5 100 −710−710 0 −780 70 70 44 T 10 85 5 20 c 2.5 145 −720 −710 10 −780 60 70

TABLE 17 Coating SWAAT-1000 h CCT-1000 h Heat Acrylic Maximum Maximumexchange Refrigerant resin Fin corrosion corrosion core tube Si KZnF₃binder Amount material depth of tube Corrosion depth of tube CorrosionNo. Alloy (%) (%) (%) (g/m³) Alloy (mm) of fin (mm) of fin 1 A 20 60 2013 c 0.05 Excellent Excellent 0.05 Excellent Excellent 2 B 20 60 20 13 c0.05 Excellent Excellent 0.05 Excellent Excellent 3 C 20 60 20 13 c 0.05Excellent Excellent 0.05 Excellent Excellent 4 D 20 60 20 13 c 0.05Excellent Excellent 0.05 Excellent Excellent 5 E 20 60 20 13 c 0.03Excellent Excellent 0.03 Excellent Excellent 6 F 20 60 20 13 c 0.03Excellent Excellent 0.03 Excellent Excellent 7 G 20 60 20 13 c 0.04Excellent Excellent 0.04 Excellent Excellent 8 H 20 60 20 13 c 0.03Excellent Excellent 0.03 Excellent Excellent 9 I 20 60 20 13 c 0.03Excellent Excellent 0.03 Excellent Excellent 10 J 20 60 20 13 c 0.03Excellent Excellent 0.03 Excellent Excellent 11 K 20 60 20 13 c 0.03Excellent Excellent 0.03 Excellent Excellent 12 L 20 60 20 13 c 0.05Excellent Excellent 0.05 Excellent Excellent 13 D 20 60 20 13 a 0.05Excellent Excellent 0.05 Excellent Excellent 14 D 20 60 20 13 b 0.05Excellent Good 0.05 Excellent Excellent 15 D 20 60 20 13 d 0.05Excellent Excellent 0.05 Excellent Excellent 16 D 20 60 20 13 e 0.05Excellent Excellent 0.05 Excellent Excellent 17 D 20 60 20 13 f 0.05Excellent Excellent 0.05 Excellent Excellent 18 D 20 60 20 13 g 0.05Excellent Excellent 0.05 Excellent Excellent 19 D 20 60 20 13 h 0.05Excellent Excellent 0.05 Excellent Excellent 20 D 20 60 20 13 i 0.05Excellent Excellent 0.05 Excellent Excellent 21 D 20 60 20 13 j 0.05Excellent Excellent 0.05 Excellent Excellent 22 D 20 60 20 13 k 0.05Excellent Excellent 0.05 Excellent Excellent 23 D 20 60 20 13 l 0.05Excellent Excellent 0.05 Excellent Excellent 24 D 20 60 10 13 c 0.05Excellent Excellent 0.05 Excellent Excellent

TABLE 18 Coating SWAAT-1000 h CCT-1000 h Heat Acrylic Maximum Maximumexchange Refrigerant resin corrosion corrosion core tube Si KZnF₃ binderAmount Fin material depth of tube Corrosion depth of tube Corrosion No.Alloy (%) (%) (%) (g/m³) Alloy (mm) of fin (mm) of fin 25 D 20 60 20 13m 0.30 Bad Excellent 0.30 Bad Excellent 26 D 20 60 20 13 n 0.05Excellent Bad 0.05 Excellent Bad 27 D 20 60 20 13 o 0.05 ExcellentExcellent 0.05 Excellent Excellent 28 D 20 60 20 13 p 0.30 Bad Excellent0.30 Bad Excellent 29 D 20 60 20 13 q 0.05 Excellent Bad 0.05 ExcellentBad 30 D 20 60 20 13 r 0.05 Excellent Excellent 0.05 Excellent Excellent31 D 20 60 20 13 s 0.30 Bad Bad 0.30 Bad Bad 32 D 20 60 20 13 t 0.05Excellent Excellent 0.05 Excellent Excellent 33 D 20 60 20 13 u 0.05Excellent Excellent 0.05 Excellent Excellent 34 D 20 60 20 13 v 0.05Excellent Excellent 0.05 Excellent Excellent 35 D 20 60 20 13 w 0.05Excellent Bad 0.05 Excellent Bad 36 D 20 60 20 13 x 0.05 Excellent Bad0.05 Excellent Bad 37 D 20 60 20 4 c 0.11 Fair Good 0.30 Bad Good 38 D20 60 20 25 c 0.05 Excellent Excellent 0.05 Excellent Excellent 39 D 2473 3 13 c 0.05 Excellent Excellent 0.05 Excellent Excellent 40 D 16 4935 13 c 0.05 Excellent Good 0.21 Bad Good 41 D 5 75 20 13 c 0.05Excellent Excellent 0.05 Excellent Excellent 42 D 45 35 20 13 c 0.08Good Good 0.30 Bad Good 43 T 20 60 20 13 c 0.30 Bad Good 0.30 Bad Good44 T 10 85 5 20 c 0.22 Bad Good 0.30 Bad Good

The results of Tests 4 to 7 were as follows. The heat exchanger coresNo. 1 to 24 produced according to the present invention showed noleakage at the joint between the header and the refrigerant tube whensubjected to the leakage test after brazing. On the other hand, the heatexchanger cores No. 43 and 44 using Alloy T having a low Mn content asthe refrigerant tube aluminum alloy showed leakage.

The heat exchanger cores No. 1 to 24 produced according to the presentinvention showed no intergranular corrosion. On the other hand, the heatexchanger cores No. 43 and 44 using Alloy T containing Cu as therefrigerant tube aluminum alloy showed significant intergranularcorrosion.

In the heat exchanger cores No. 1 to 24 produced according to thepresent invention, a sufficient Zn diffusion layer was formed in thesurface of the refrigerant tube. Therefore, the surface of therefrigerant tube had a potential lower than that of the deep area of therefrigerant tube. The potential difference between the surface and thedeep area of the refrigerant tube was 95 to 100 mV. The potential of thefin material was also lower than that of the deep area of therefrigerant tube. A sufficient Zn diffusion layer was not formed in thesurface of the refrigerant tube in some of the heat exchanger cores No.25 to 44 produced under conditions that do not meet the requirements ofthe present invention. In this case, a sufficient potential differencewas not obtained between the surface and the deep area of therefrigerant tube. In the heat exchanger cores No. 43 and 44 using AlloyT containing Cu as the refrigerant tube aluminum alloy, since thepotential-decreasing effect of Zn was counterbalanced, the surface ofthe refrigerant tube had a potential equal to or slightly lower thanthat of the deep area of the refrigerant tube, although a sufficient Zndiffusion layer was formed.

When subjecting the heat exchanger cores No. 1 to 24 produced accordingto the present invention to the SWAAT test, the maximum corrosion depthwas small (i.e., excellent corrosion resistance was obtained) since asufficient potential difference was obtained between the surface and thedeep area of the refrigerant tube. In the SWAAT test, since the finexhibits a sacrificial anode effect, corrosion of the fin materialdiffers depending on the potential difference between the surface of therefrigerant tube and the fin material. In the heat exchanger cores No. 1to 24 produced according to the present invention, the fin material wascorroded to no or only a small extent due to an appropriate potentialdifference between the surface of the refrigerant tube and the finmaterial. Moreover, the potential of the fin material was lower thanthat of the deep area of the refrigerant tube. Therefore, the finmaterial did not accelerate corrosion of the refrigerant tube as acathode.

With regard to the heat exchanger cores No. 25 to 44 produced underconditions that do not meet the requirements of the present invention,the maximum corrosion depth was large in the heat exchanger cores No.25, 28, 31, 37, 43, and 44 in which a sufficient potential differencewas not obtained between the surface and the deep area of therefrigerant tube, or the potential of the fin material was higher thanthat of the deep area of the refrigerant tube. The fin of Heat exchangercore No. 26 using Alloy n having a high Zn content as the fin materialshowed significant corrosion since the potential of the fin material wassignificant lower than that of the surface of the refrigerant tube. Inthe heat exchanger cores No. 29, 31, 35, and 36 using Alloy q having ahigh Fe content, Alloy s having a high Cu content, Alloy w having a highIn content, or Alloy x having a high Sn content as the fin material, thefin showed significant corrosion due to inferior self-corrosionresistance.

When subjecting the heat exchanger cores No. 1 to 24 produced accordingto the present invention to the CCT test (the CCT test is similar to theactual environment due to the drying step; however, the fin may notexhibit a sacrificial anode effect), the maximum corrosion depth of therefrigerant tube was small (i.e., excellent corrosion resistance wasobtained) since a sufficient potential difference was obtained betweenthe surface and the deep area of the refrigerant tube. The fin materialwas corroded to no or only a small extent.

Regarding the heat exchanger cores No. 25 to 44 produced underconditions that do not meet the requirements of the present invention,the maximum corrosion depth of the refrigerant tube was large when thepotential difference between the surface and the deep area of therefrigerant tube was insufficient. The same tendency as that of theSWAAT test was observed for corrosion of the fin material. The heatexchanger cores No. 27, 30, 32-34, 38, 39, and 41 showed excellentcorrosion resistance results. However, a problem occurred when producingthe heat exchanger core (see Table 12).

Obviously, numerous modifications and variations of the presentinvention are possible in the light of the above teachings. It istherefore to be understood that within the scope of the appended claims,the invention may be practised otherwise than as specifically describedherein.

1. An aluminum alloy heat exchanger that is produced by applying acoating material that is prepared by adding a binder to a mixture of anSi powder and a Zn-containing compound flux powder to a surface of analuminum alloy refrigerant tube, assembling a bare fin that is formed ofan Al—Mn—Zn alloy with the refrigerant tube, and brazing the refrigeranttube and the bare fin by heating in an atmosphere-controlled furnace,the refrigerant tube being an aluminum alloy extruded product thatcomprises 0.5 to 1.7% (mass %, hereinafter the same) of Mn, less than0.10% of Cu, and less than 0.10% of Si, with the balance being Al andunavoidable impurities, a mixing ratio of the Si powder to theZn-containing compound flux powder being 10:90 to 40:60, the binderbeing added in an amount of 5 to 40% based on the total amount of thecoating material, the coating material being applied to an outer surfaceof the refrigerant tube so that the total amount of the Si powder andthe Zn-containing compound flux powder is 5 to 30 g/m², the surface ofthe refrigerant tube subjected to brazing having a potential lower thanthat of an area of the refrigerant tube by 20 to 200 mV, and a potentialof the fin being lower than that of the area of the refrigerant tubethat is deeper than the diffusion depth of Si and Zn.
 2. The aluminumalloy heat exchanger according to claim 1, wherein the aluminum alloyextruded product further comprises at least one of 0.30% or less of Ti,0.10% or less of Sr, and 0.3% or less of Zr.
 3. The aluminum alloy heatexchanger according to claim 1, wherein the Zn-containing compound fluxpowder is KZnF₃.
 4. The aluminum alloy heat exchanger according to claim1, wherein the Al—Mn—Zn alloy comprises 0.1 to 1.8% of Mn, 0.8 to 3.0%of Zn, and at least one of 0.1 to 1.2% of Si, 0.01 to 0.8% of Fe, 0.05to 0.5% of Mg, 0.3% or less of Cu, 0.3% or less of Cr, 0.3% or less ofZr, and 0.3% or less of Ti, with the balance being Al and unavoidableimpurities.
 5. The aluminum alloy heat exchanger according to claim 4,wherein the Al—Mn—Zn alloy further comprises at least one of 0.001 to0.10% of In and 0.001 to 0.10% of Sn.
 6. A method of producing arefrigerant tube that is used for the aluminum alloy heat exchangeraccording to claim 1, the method comprising subjecting an ingot of thealuminum alloy that forms the refrigerant tube to a homogenization heattreatment that holds the ingot at 400 to 650° C. for 4 hours or more,and hot-extruding the ingot.
 7. A method of producing a refrigerant tubethat is used for the aluminum alloy heat exchanger according to claim 1,the method comprising subjecting an ingot of the aluminum alloy thatforms the refrigerant tube to a homogenization heat treatment, andhot-extruding the ingot, the homogenization heat treatment including afirst-stage heat treatment that holds the ingot at 570 to 650° C. for 2hours or more, and a second-stage heat treatment that holds the ingot at400 to 550° C. for 3 hours or more.
 8. A method of producing arefrigerant tube that is used for the aluminum alloy heat exchangeraccording to claim 1, the method comprising subjecting an ingot of thealuminum alloy that forms the refrigerant tube to a homogenization heattreatment, and hot-extruding the ingot, the homogenization heattreatment including a first-stage heat treatment that holds the ingot at570 to 650° C. for 2 hours or more, followed by cooling the ingot to200° C. or less, and a second-stage heat treatment that holds the ingotat 400 to 550° C. for 3 hours or more.
 9. A method of producing arefrigerant tube that is used for the aluminum alloy heat exchangeraccording to claim 1, the method comprising subjecting an ingot of thealuminum alloy that forms the refrigerant tube to a homogenization heattreatment, and hot-extruding the ingot, the homogenization heattreatment including a first-stage heat treatment that holds the ingot at570 to 650° C. for 2 hours or more, and a second-stage heat treatmentthat holds the ingot at 400 to 550° C. for 3 hours or more.
 10. A methodof producing a refrigerant tube that is used for the aluminum alloy heatexchanger according to claim 5, the method comprising subjecting aningot of the aluminum alloy that forms the refrigerant tube to ahomogenization heat treatment, and hot-extruding the ingot, thehomogenization heat treatment including a first-stage heat treatmentthat holds the ingot at 570 to 650° C. for 2 hours or more, and asecond-stage heat treatment that holds the ingot at 400 to 550° C. for 3hours or more.
 11. A method of producing a refrigerant tube that is usedfor the aluminum alloy heat exchanger according to claim 9, the methodcomprising subjecting an ingot of the aluminum alloy that forms therefrigerant tube to a homogenization heat treatment, and hot-extrudingthe ingot, the homogenization heat treatment including a first-stageheat treatment that holds the ingot at 570 to 650° C. for 2 hours ormore, followed by cooling the ingot to 200° C. or less, and asecond-stage heat treatment that holds the ingot at 400 to 550° C. for 3hours or more.
 12. A method of producing a refrigerant tube that is usedfor an aluminum alloy heat exchanger, the method comprising subjectingan ingot of the aluminum alloy described in claim 5 that forms therefrigerant tube to a homogenization heat treatment, and hot-extrudingthe ingot, the homogenization heat treatment including a first-stageheat treatment that holds the ingot at 570 to 650° C. for 2 hours ormore, followed by cooling the ingot to 200° C. or less, and asecond-stage heat treatment that holds the ingot at 400 to 550° C. for 3hours or more.